This invention relates to a microwave oven or range. More particularly, this invention relates to a microwave oven having an infrared detector which does not require any phase adjustment to accommodate frequency variations caused by the fluctuations in the power supply frequency, which is a relatively small and thin structure, and which is capable of accurately detecting temperatures with high reliability for a long period of time.
It is important to determine the exact temperature of an object to be heated for the purpose of operating a microwave oven effectively. For this purpose, there has been proposed a microwave oven having an infrared detector for detecting the temperature of an object to be heated in a non-contacting manner. In order to detect the temperature by using a pyroelectric infrared detector, it is necessary to alternately interrupt incident infrared rays mechanically by a ray-chopper to produce an alternating signal. This requires a mechanism for alternately driving the chopper. For such alternate driving, a synchronous motor has been used because it has good rotational frequency stability and is relatively inexpensive. Thus, a mechanism has been used in which blades are mounted on the output shaft of such a synchronous motor.
However, a synchronous motor has the drawback that a low start up torque is developed in response to a stable rotational frequency since rotation is synchronous with the power frequency.
In order to remove this drawback, a synchronous motor is required to be connected to a reduction gear or such a structure that permits slippage between the rotation of the blades and the motor shaft upon start-up. However, such a motor with a slippage structure has the disadvantage that it lacks reliability for a prolonged usage. Also, the rotational frequency of a synchronous motor is different depending upon the power frequency. Since a pyroelectric infrared detector has a frequency characteristic when a microwave oven is from an area having one power frequency to an area having a different power frequency, a sensitivity compensation must be made. Further, in cases where the signals from the pyroelectric infrared detector are treated by synchronous detection, the blades of the chopper must be phased with the infrared input, namely, mechanical adjustment is required when the microwave oven is moved from one area to another where different power frequencies are used.
Finally, the aforementioned chopper-driving mechanism is structurally complicated due to unavoidable factors such as the size of the synchronous motor and others, leading to difficulty of reduction in size.